1. Field of the Invention
This invention relates to a method and system for improving immunity to jamming in systems utilizing the global positioning system (GPS).
2. Brief Description of the Prior Art
Operations utilizing signals received from the GPS have been universally utilized both in conjunction with commercial and military applications. GPS and inertial measurement units (IMUs) have been combined or coupled together in the prior art to provide more effective navigation with the GPS correcting the IMU. This combination provides a synergistic result in that the effective bandwidth of the system can be reduced in an optimum fashion to provide improved tracking.
A problem with the use of the GPS is that it can easily be jammed. Systems have been devised in the prior art to minimize the detrimental effects of GPS jamming. However, jamming and the possibility of jamming continue and still can present navigation and other problems. Accordingly, systems which provide increased immunity against such jamming are highly desirable and, in fact, necessary in many applications. It follows that any improvement in immunity against jamming by further reduction in bandwidth is highly desirable.
Traditional GPS receivers contain intermediate tracking loops which operate well at high carrier to noise density (C/NO) (&gt;23 dB-Hz) but break down at low C/NO conditions. Delay lock loops (DLLs) are used for tracking code phase and phase lock loops (Costas loops) are used to track carrier phase. The outputs for each loop type are used to generate pseudo range and delta (.DELTA.) range measurements respectively, which are used by a Kalman filter in formation of range and delta (.DELTA.) range residuals (error signals input to the Kalman filter). The noise bandwidths for these loops are usually on the order of 0.025 and 1 Hz for the DLL and Costas loops respectively.
The following list enumerates the problems associated with the traditional solutions to the jamming problem:
1. Under jamming conditions (with low C/NO), narrow tracking loop bandwidths are usually employed, resulting in temporally correlated noise which is a suboptimal solution when viewed from the point of view of the Kalman filter. These narrow bandwidths also produce a correlation effect between measurements and process noise which tends to have destabilizing effects on system performance. PA0 2. As jamming increases, the carrier tracking loop breaks down somewhere in the C/NO neighborhood of 18 dB-Hz. The fundamental reason for the loop break downs is that the signal to noise ratio (SNR) into the (Costas) loop, which is the product of carrier power/noise density (C/NO) and the coherent integration time ((C/NO).multidot.T.sub.i =SNR which is the signal to noise ratio prior to the squaring operation). When this happens, the information loss through traditional non-linear loop error discriminants, the arctan function being an example, becomes prohibitive, resulting in virtually no restoring force to any loop perturbation. The only way to recover signal to noise ratio (SNR) is to integrate longer and use narrow bandwidths, resulting in correlated measurement problems described in (1.) above. The loss of carrier loop measurements is key since these measurements provide the most accurate source of information necessary to maintain the INS system alignment (i.e., system errors small). PA0 3. Traditional schemes that employ intermediate tracking loops are somewhat sluggish to reacquire (and acquire) the GPS signals following a jamout period. This is because a certain amount of time must be allocated for the tracking loops to pull in to lock. This time is somewhat large for narrow tracking loop bandwidths which use long time constants.
Intermediate tracking loops employ fixed gains which do not "adapt" to time varying signal conditions (e.g., jamming or loss of data to be used for data wipe off (DWO)). Data wipe off is a method used to improve signal to noise ratio. Since I and Q data input to the baseband algorithm is bi-phase shift keyed (BPSK) modulated at a 50 Hz rate, the signal polarity can therefore change at a 50 Hz rate (e.g. Q=.+-.A sin .THETA.). Clearly, when this happens, coherently adding I or Q signal samples over time can result in signal cancellation. DWO is a technique which uses a priori estimates of the 50 Hz data stream to remove this effect, thus allowing coherent integration and its benefits. DWO is normally accomplished by demodulation of the data prior to jamout and then storing of the data for later use. This demodulation must take place for the primary purpose of navigation, so the availability of most of the required data is a given. When this happens, the tracking loops are in a state of random walk with a divergence rate determined by the fixed gains employed. This creates measurement, editing and recovery problems when the signal conditions again become favorable. This creates a very suboptimal situation in which data used for DWO is misestimated.
The standard performance metrics when discussing GPS operation under jamming conditions are receiver threshold jamming to signal levels (J/S) and carrier to noise density (C/NO). Under conditions of wideband jamming, C/NO (dB-Hz)=73-J/S (dB). Therefore, under conditions of high J/S, the signal to noise ratio (SNR) decreases proportionally for signals delivered to baseband signal processing algorithms (intermediate baseband tracking loops).
An approach to reducing tracking loop noise bandwidth in order to navigate during modest amounts of jamming is inertial measurement unit (IMU) inertial aiding techniques of code and carrier tracking loops. Assuming that a nulling antenna is not used, tracking loops currently phase lock and code lock to J/S levels of approximately 55 dB and 60 dB respectively. The outputs of these loops produce range and delta (.DELTA.) range measurements to a Kalman filter (KF), generally an 18 state navigation Kalman filter though it should be understood that the number of states of the Kalman filter is optional. Above these jamming levels, complete loss of GPS measurements occurs, resulting in the inertial navigation system drifting in a divergent fashion.